1. Field of the Invention
In certain types of optical display devices containing liquid crystals, it is necessary to induce alignment of the liquid crystal molecules such that the directors, or long axes, of the molecules are perpendicular to the surface of the substrate upon which the liquid crystal is brought in contact. In those instances where the device is a prefabricated cell whose elements are joined together by an adhesive seal or by a high temperature assembly method such as a glass frit seal, the inducement of perpendicular alignment of liquid crystals subsequently placed therein is particularly difficult. This invention is concerned with the solution of this problem and generally with perpendicular alignment of liquid crystals on substrate surfaces.
2. Description of the Prior Art
Many different techniques have been used for surface perpendicular (homeotropic) alignment of liquid crystals, as have been summarized by others (see: E.L. Williams, "Liquid Crystals For Electron Devices," in Noyes Data Corp., 1975, pp. 193-211; J. D. Margerum and L. J. Miller, J. Colloid and Interface Sci, 1977, pp. 562-565; and W. H. deJeu, "Physical Properties of Liquid Crystalline Material," Gordon and Breach, 1980, pp. 16-23). Three general types of methods are used for homeotropic alignment. One is the use of alignment dopants mixed into the liquid crystal mixture, such as that described in U.S. Pat. No. 3,656,834 issued to I. Haller; in U.S. Pat. No. 3,698,449 issued to Sorkin; in U.S. Pat. No. 3,809,456 issued to J. E. Goldmacher; and in Canadian Pat. No. 1061099 issued to D. Skelly. The problems with using these dopants is that they alter some of the liquid crystal characteristics, such as conductivity, they do not exhibit long-term stability, and the alignment may be temperature-dependent due to temperature effects on the partition of the dopant between the liquid crystal and the substrate surface. The second method uses surface coated films for homeotropic alignment, as reported by W. E. L. Haas in U.S. Pat. Nos. 3,687,515 and 3,803,050. The polymer coatings described in the '515 and '050 Haas patents often are lacking in long-term stability and uniformity and, as discussed below, are adversely affected by high temperature device fabrication methods. The third alignment technique, surface alteration by chemical reaction with the surface, is the most inert toward the liquid crystal and includes the use of alkoxysilanes as described by F. J. Kahn, in Applied Physics Letters, 22, 111, 1973 and the use of alcohol baths as disclosed and claimed by U.S. Pat. No. 4,022,934 issued to co-applicant Leroy J. Miller.
The Miller '934 patent is very relevant to the present invention in that it also discloses the use of long-chain alcohols on substrate surfaces to induce surface perpendicular alignment. Unlike the present invention, the Miller process uses hot, liquid baths of long-chain alcohols and amines to apply alcohol coatings to the surface of the liquid crystal substrates. The substrate surfaces are totally immersed in the baths and subsequently washed with a solvent to remove excess alcohol prior to their use for containment of liquid crystals. While the '934 Miller process is suitable for use with demountable cells, this method is not suitable for use with prefabricated displays because it would be difficult to remove the excess alcohol and because high temperature prefabrication techniques such a glass frit sealing destroy the organic alignment-inducing coating.
Therefore, there is a need for a method of inducing perpendicular alignment of liquid crystals on the surfaces of substrates previously fabricated into liquid crystal cells by high temperature assembly techniques or by adhesive sealing methods.